Broadband wavelength multiplexing and demultiplexing filter and optical splitter with optical signal multiplexing and demultiplexing function

ABSTRACT

Two Mach-Zehnder optical interferometer circuits  13   a  and  13   b  are accurately point-symmetrically connected to each other to form a first point-symmetrically connected optical interferometer circuit  5  constituting a light input side circuit  1 . Optical signals having a plurality of wavelengths are input to a light input terminal  17 . A second point-symmetrically connected optical interferometer circuit  7  having the same functional structure as the first point-symmetrically connected optical interferometer circuit  5  is connected to a through port  18 , which is an output terminal of the light input side circuit  1 , as a first light output side circuit  2 . A cross port  19 , which is the other output terminal of the light input side circuit  1 , is connected to a second light output side circuit  3  having at least one of Mach-Zehnder optical interferometer circuits  13   c  and  13   d  whose transmittance characteristics are different from those of the Mach-Zehnder optical interferometer circuits  13   a  and  13   b.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 12/324,205,filed Nov. 26, 2008, which is a division of and is based upon and claimsthe benefit of priority under 35 U.S.C. §120 for U.S. application Ser.No. 10/560,132, filed Dec. 9, 2005, which is a National Stage ofPCT/JP05/01295 filed Jan. 25, 2005, and claims the benefit of priorityunder 35 U.S.C. §119 from Japanese Patent Application No. 2004-016589,filed Jan. 26, 2004 and Japanese Patent Application No. 2004-188365,filed Jun. 25, 2004 and Japanese Patent Application No. 2004-222250,filed Jul. 29, 2004. The entire contents of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a broadband wavelength multiplexing anddemultiplexing filter and an optical splitter with an optical signalmultiplexing and demultiplexing function used for, for example, anoptical communication field.

BACKGROUND ART

In recent years, an Internet has been popularized in home, andhigh-speed and cheap communication services, in which a constantconnection has been made by ADSL or FTTH, of which a transmission speedis several Mbps, and of which a monthly subscription is several tens ofdollars, have been provided. Further, with the spread of broadbandcommunication, a large amount of contents have been distributed, and avideo chat has come into wide use.

With the spread of broadband communication, a B-PON system capable ofproviding high-speed data communication having a data transmission speedof 100 Mbps at a peak time and of simultaneously distributingmulti-channel (for example, a maximum of 500 channels) image signalsusing one optical fiber has been used as an international standard ofITU-T. The structure of this system is shown in FIG. 26 (for example,see an outline of the B-PON system described in the home page of anNTT-AS institute).

Further, in FIG. 26, a communication station 40 is connected to a userhome 41 through a communication optical fiber 45. In FIG. 26, B-ONUindicates a data-based subscriber station apparatus for data, WDMindicates an optical coupler, V-ONU indicates an image-based subscriberstation apparatus, NE-OSS indicates a data-based monitor controlapparatus, B-OLT indicates a data-based service station apparatus, EMDXindicates an Ether demultiplexing apparatus, and V-OLT indicates animage-based service station apparatus.

The salient characteristic of this system is that one optical wavelengthis added to two wavelengths used for the transmission and reception ofhigh-speed data by an additional optical wavelength arrangement and thusthe distributed multi-channel images can be simultaneously seen. Ingeneral, wavelengths of high-speed data communication signals to betransmitted and received are included in a wavelength band of 1.49 μm(1.48 μm to 1.50 μm) or a wavelength band of 1.31 μm (1.26 μm to 1.36μm), and an image is distributed at a wavelength band of 1.55 μm (1.55μm to 1.56 μm), which is an amplifying band of EDFA.

Since optical signals having three types of wavelengths are transmittedthrough one optical fiber, the B-PON system requires a wavelengthmultiplexing and demultiplexing function having an optical signalmultiplexing and demultiplexing function (the multiplexing anddemultiplexing of optical signals having a plurality of wavelengths) anda function of distributing image signals with the same intensity.Conventionally, a dielectric multi-layer filter has been used as anoptical component having the wavelength multiplexing and demultiplexingfunction. An optical component having the wavelength multiplexing anddemultiplexing function requires a broadband wavelength multiplexing anddemultiplexing characteristic and a high-isolation characteristicgreater than, for example, 25 dB, and a development of such a broadbandwavelength multiplexing and demultiplexing filter is demanded.

Further, since optical signals having three types of wavelengths aretransmitted through one optical fiber, the B-PON system requires anoptical signal multiplexing and demultiplexing device having an opticalsignal multiplexing and demultiplexing function (the multiplexing anddemultiplexing of optical signals having a plurality of wavelengths) andan optical splitter for splitting image signals at the same intensity.

Accordingly, an object of the present invention is to provide abroadband wavelength multiplexing and demultiplexing filters improvedfor or capable of meeting the above-mentioned requirements. In addition,another object of the present invention is to provide an opticalsplitter with an optical signal multiplexing and demultiplexingfunction, which is suitable for a B-PON system, etc.

DISCLOSURE OF INVENTION

According to a first aspect, the present invention has a followingstructure. That is, a broadband wavelength multiplexing anddemultiplexing filter according to an embodiment of the presentinvention comprises: Mach-Zehnder optical interferometer circuits eachhaving directional couplers formed on a substrate by a first opticalwaveguide and a second optical waveguide provided in parallel to eachother with a gap in a lengthwise direction of the optical waveguidestherebetween, and a phase part interposed between the directionalcouplers; a first point-symmetrically connected optical interferometercircuit formed by accurately point-symmetrically connecting two equalMach-Zehnder optical interferometer circuits in series; and a lightinput side circuit formed by connecting one or more firstpoint-symmetrically connected optical interferometer circuits in series,wherein a light input terminal of a first optical waveguide of the lightinput side circuit is composed of an input port for optical signalshaving a plurality of wavelengths, and an output terminal of the firstoptical waveguide is composed of a through port; an output terminal of asecond optical waveguide of the light input side circuit is composed ofa cross port; a first light output side circuit formed by seriallyconnecting one or more second point-symmetrically connected opticalinterferometer circuits having the same functional structure as thefirst point-symmetrically connected optical interferometer circuit isconnected to the through port; and a second light output side circuitincluding one or more Mach-Zehnder optical interferometer circuitshaving transmission characteristics different from those of theMach-Zehnder optical interferometer circuits constituting the first andthe second point-symmetrically connected optical interferometer circuitsis connected to the cross port.

Further, a broadband wavelength multiplexing and demultiplexing filteraccording to another embodiment of the present invention comprises:Mach-Zehnder optical interferometer circuits each having two directionalcouplers on a substrate, each directional coupler formed by a firstoptical waveguide and a second optical waveguide provided in parallel toeach other with a gap therebetween, and a phase-part-intervention-typepoint-symmetrically connected optical interferometer circuit formed bypoint-symmetrically arranging two equal Mach-Zehnder opticalinterferometer circuits in series and connecting them to each other witha phase part for generating a predetermined phase change interposedtherebetween, two equal phase-part-intervention-type point-symmetricallyconnected optical interferometer circuits being accuratelypoint-symmetrically connected in series; wherein the Mach-Zehnderoptical interferometer circuits have equal directional couplers, andthese directional couplers are connected in series to each other with asecond phase part for generating a phase change different from that inthe phase part interposed therebetween.

Furthermore, an optical splitter with an optical signal multiplexing anddemultiplexing function according to still another embodiment of thepresent invention comprises: an optical waveguide circuit formed on asubstrate, wherein the optical waveguide circuit comprises: an opticalsplitter for splitting an optical signal input from a light input portprovided at one end of the optical waveguide circuit into a plurality ofoptical signals having the same intensity and for outputting them from aplurality of light output ports; and a plurality of optical signalmultiplexing and demultiplexing devices arranged in parallel to eachother, each being provided with two light input ports and having afunction of demultiplexing optical signals having different wavelengthsinput from the light input ports, wherein one input port of each of theoptical signal multiplexing and demultiplexing devices is connected tothe corresponding light output port of the optical splitter; the otherlight input port of each of the optical signal multiplexing anddemultiplexing devices is provided at one end side of the opticalwaveguide circuit to be parallel to the light input port of the opticalsplitter; and a multiplexed optical signal output port of each of theoptical signal multiplexing and demultiplexing devices is provided at anend portion side other than a region where the light input port of theoptical waveguide circuit is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an essential constitutional view schematically illustrating acircuit structure of a first embodiment of a broadband wavelengthmultiplexing and demultiplexing filter according to the presentinvention;

FIG. 2 is an explanatory diagram schematically illustrating a circuitstructure of a point-symmetrically connected optical interferometercircuit;

FIG. 3 is an explanatory diagram schematically illustrating thestructure of a directional coupler;

FIG. 4A is a graph showing a result of the characteristic simulation ofthe point-symmetrically connected optical interferometer circuit (apoint-symmetrically connected optical interferometer circuit accordingto the first embodiment of the present invention) under a condition inwhich a coupling wavelength of the directional coupler is 924 μm;

FIG. 4B is a graph showing a result of the characteristic simulation ofthe point-symmetrically connected optical interferometer circuit (thepoint-symmetrically connected optical interferometer circuit accordingto the first embodiment of the present invention) under the condition inwhich the coupling wavelength of the directional coupler is 2184 μm;

FIG. 5 is a graph showing a transmission characteristic simulationresults of optical signals respectively output from a through port and across port of the point-symmetrically connected optical interferometercircuit according to the first embodiment of the present invention;

FIG. 6 is a graph showing a transmission characteristic of a throughport propagating optical signal of the broadband wavelength multiplexingand demultiplexing filter according to the first embodiment of thepresent invention, compared to that of a through port propagatingoptical signal of one point-symmetrically connected opticalinterferometer circuit;

FIG. 7 is a graph showing a transmission characteristic of a crosspropagating optical signal of the broadband wavelength multiplexing anddemultiplexing filter according to the first embodiment of the presentinvention, compared to those of cross propagating optical signals of thepoint-symmetrically connected optical interferometer circuit and aMach-Zehnder optical interferometer circuit both included in thebroadband wavelength multiplexing and demultiplexing filter;

FIG. 8 is a graph showing transmission characteristics of the throughpropagating optical signal and the cross propagating optical signal ofthe broadband wavelength multiplexing and demultiplexing filteraccording to the first embodiment of the present invention;

FIG. 9 is a view schematically illustrating a circuit structure of asecond embodiment of the broadband wavelength multiplexing anddemultiplexing filter according to the present invention;

FIG. 10 is a graph showing the transmission characteristics of a throughpropagating optical signal and a cross propagating optical signal of thebroadband wavelength multiplexing and demultiplexing filter according tothe second embodiment of the present invention;

FIG. 11 is an essential constitutional view schematically illustrating acircuit structure of a third embodiment of the broadband wavelengthmultiplexing and demultiplexing filter of the present invention;

FIG. 12A is a graph showing an optical coupling characteristic of thebroadband wavelength multiplexing and demultiplexing filter shown inFIG. 11;

FIG. 12B is a graph showing a wavelength characteristic of thetransmittance of the broadband wavelength multiplexing anddemultiplexing filter shown in FIG. 11;

FIG. 13 is a view schematically illustrating a circuit structure of afourth embodiment of the broadband wavelength multiplexing anddemultiplexing filter of the present invention;

FIG. 14 is a graph showing a wavelength characteristic of thetransmittance of light output from a through port of the wavelengthmultiplexing and demultiplexing filter shown in FIG. 13;

FIG. 15 is a graph showing a wavelength characteristic of thetransmittance of light output from a cross port of the wavelengthmultiplexing and demultiplexing filter shown in FIG. 13;

FIG. 16 is a graph showing a wavelength characteristic of thetransmittance of the broadband wavelength multiplexing anddemultiplexing filter shown in FIG. 13;

FIG. 17 is an essential constitutional view schematically illustrating acircuit structure of a fifth embodiment of the broadband wavelengthmultiplexing and demultiplexing filter of the present invention;

FIG. 18 is a graph showing a wavelength characteristic of thetransmittance of the broadband wavelength multiplexing anddemultiplexing filter shown in FIG. 17;

FIG. 19 is an essential constitutional view schematically illustrating acircuit structure of a broadband optical splitter/router of a sixthembodiment using the broadband wavelength multiplexing anddemultiplexing filter according to the present invention;

FIG. 20A is a graph showing a transmission characteristic of an opticalsplitter function of the broadband optical splitter/router shown in FIG.19;

FIG. 20B is a graph showing a transmission characteristic of eachchannel with respect to an optical router function;

FIG. 21 is an explanatory diagram illustrating a structure of an opticalsplitter with a broadband wavelength multiplexing and demultiplexingfilter function of a seventh embodiment using the broadband wavelengthmultiplexing and demultiplexing filter according to the presentinvention;

FIG. 22 is a graph showing a wavelength characteristic of thetransmittance of the optical splitter with a broadband wavelengthmultiplexing and demultiplexing filter function shown in FIG. 21;

FIG. 23 is an explanatory diagram schematically illustrating a circuitstructure of an optical splitter with an optical signal multiplexing anddemultiplexing function according to an eighth embodiment of the presentinvention;

FIG. 24 is an explanatory diagram illustrating an optical module formedby applying the optical splitter with an optical signal multiplexing anddemultiplexing function of the eighth embodiment;

FIG. 25 is a graph showing a transmission loss characteristic of anoptical signal input from one of input ports of an optical signalmultiplexing and demultiplexing device provided at one end side of anoptical waveguide circuit, in the optical splitter with an opticalsignal multiplexing and demultiplexing function of the eighthembodiment;

FIG. 26 is an explanatory diagram illustrating the structure of aconventional B-PON system;

FIG. 27 is a graph showing a transmission characteristic and areflection characteristic of a dielectric multi-layer filter; and

FIG. 28 is an explanatory diagram illustrating the structure of aconventional module applied to the B-PON system.

BEST MODE FOR CARRYING OUT THE INVENTION

A circuit having a structure shown in FIG. 2 has been known as anexample of an optical component having a broadband wavelengthmultiplexing and demultiplexing characteristic (for example, seeJinguji, et al., “Two-port optical wavelength circuits composed ofcascaded Mach-Zehnder interferometers with point-symmetricalconfigurations”, J. Lightwave Technol., vol. 14, No. 10, pp. 2301-2310(1996)).

This circuit has two equal Mach-Zehnder optical interferometer circuits13 (13 a and 13 b). Each Mach-Zehnder optical interferometer circuit 13(13 a or 13 b) has directional couplers 6 each composed of a firstoptical waveguide 11 and a second optical waveguide 12 provided inparallel to the first optical waveguide 11 with a gap therebetween, andthe directional couplers 6 are respectively arranged in the lengthwisedirection of the optical waveguide with a gap therebetween. These twoMach-Zehnder optical interferometer circuits 13 (13 a and 13 b) areconnected in series to each other so as to be exactly point-symmetricwith respect to a central point A of the connection to form apoint-symmetrically connected optical interferometer circuit 5.

That is, the arrangement pitch between the directional couplers 6 in oneMach-Zehnder optical interferometer circuit 13 (13 a or 13 b) is equalto that of the other Mach-Zehnder optical interferometer circuit 13 (13a or 13 b), and in one Mach-Zehnder optical interferometer circuit 13 a,the length of a phase part 9 of the first optical waveguide 11 is largerthan that of the second optical waveguide 12 by a predetermined length.On the other side, in the other Mach-Zehnder optical interferometercircuit 13 b, the length of the phase part 9 of the second opticalwaveguide 12 is larger than that of the first optical waveguide 11 bythe predetermined length.

Therefore, as shown in FIG. 27, in a conventional dielectric multi-layerfilter applied to a B-PON system, the isolation of a non-reflection bandof reflected light (which is represented by a feature line b) to atransmission band of transmission light (which is represented by afeature line a) is high (see a character ‘A’ of FIG. 27). On the otherhand, the isolation of a reflection band of the reflected light to anon-transmission band of the transmission light deteriorates largely(see a character ‘B’ of FIG. 27). These features cannot satisfy a highisolation characteristic currently being required.

Further, when the dielectric multi-layer filter is integrated into amodule, a micro optics technique using a lens system is applied thereto.Therefore, the part price is not lowered, and thus it is difficult toreduce manufacturing costs.

Furthermore, the point-symmetrically connected optical interferometercircuit shown in FIG. 2 is designed only to have a broadband wavelengthmultiplexing and demultiplexing characteristic. Therefore, it isdifficult for the circuit to obtain a high isolation characteristic.

First Embodiment

A broadband wavelength multiplexing and demultiplexing filter accordingto an embodiment of the present invention is designed to solve theabove-mentioned problems.

FIG. 1 shows a first embodiment of the broadband wavelength multiplexingand demultiplexing filter according to the present invention. Thebroadband wavelength multiplexing and demultiplexing filter of thepresent embodiment is constructed by forming an optical waveguidecircuit shown in the figure on a substrate 15. This circuit has a lightinput side circuit 1, a first light output side circuit 2, and a secondlight output side circuit 3.

The light input side circuit 1 has two Mach-Zehnder opticalinterferometer circuits 13 (13 a and 13 b) connected in series to eachother. Each Mach-Zehnder optical interferometer circuit 13 (13 a or 13b) has two directional couplers 6 each composed of the first opticalwaveguide 11 and the second optical waveguide 12 arranged in parallel tothe first optical waveguide 11 with a gap therebetween, and thedirectional couplers are separated from each other in the lengthwisedirection of the optical waveguide.

A connecting circuit between these two Mach-Zehnder opticalinterferometer circuits 13 (13 a and 13 b) is a circuit shown in FIG. 2and is referred to as the first point-symmetrically connected opticalinterferometer circuit 5 in the present embodiment. In this circuit, thearrangement pitch between the directional couplers 6 of the Mach-Zehnderoptical interferometer circuits 13 (13 a) is equal to that between thedirectional couplers 6 of the Mach-Zehnder optical interferometercircuit 13 (13 b). In addition, in the Mach-Zehnder opticalinterferometer circuit 13 a, the length of the phase part 9 of the firstoptical waveguide 11 is lager than that of the second optical waveguide12 by a set length (in the present embodiment, ΔLc), and in the otherMach-Zehnder optical interferometer circuit 13 b, the length of thephase part 9 of the second optical waveguide 12 is larger than that ofthe first optical waveguide 11 by the set length (in the presentembodiment, ΔLc).

Further, the value of the set length ΔLc is properly set in theMach-Zehnder optical interferometer circuits 13, which will be describedbelow, including the Mach-Zehnder optical interferometer circuits 13 aand 13 b constituting the first point-symmetrically connected opticalinterferometer circuit 5.

Furthermore, as shown in FIG. 1, a light input terminal 17 of the firstoptical waveguide 11 of the light input side circuit 1 is composed of aninput port for a plurality of light wavelengths. For the input port (INport), an output terminal of the first optical waveguide 11 providedwith the input port (light input terminal 17) is composed of a throughport 18, and an output terminal of the second optical waveguide 12 notprovided with the light input port of the light input side circuit 1 iscomposed of a cross port 19.

The first light output side circuit 2 is connected to the through port18 of the light input side circuit 1. The first light output sidecircuit 2 is formed of a second point-symmetrically connected opticalinterferometer circuit having the same functional structure as the firstpoint-symmetrically connected optical interferometer circuit 5.

Meanwhile, a second light output side circuit 3 is connected to thecross port 19 of the light input side circuit 1. The second light outputside circuit 3 includes Mach-Zehnder optical interferometer circuits 13(13 c and 13 d) having a different length structure (a total length ofthe phase part 9 and the two directional couplers 6 is differenttherefrom), that is, different light transmitting characteristic andwavelength multiplexing and demultiplexing characteristic, from those ofthe Mach-Zehnder optical interferometer circuits 13 (13 a and 13 b)respectively constituting the first and second point-symmetricallyconnected optical interferometer circuits 5 and 7.

These Mach-Zehnder optical interferometer circuits 13 c and 13 d areconnected at n stages (where n is an integral number equal to or greaterthan 2, and in the present embodiment, n is 2), a light input side ofthe first optical waveguide 11 of the second-stage Mach-Zehnder opticalinterferometer circuit 13 d is connected to a light output side of thesecond optical waveguide 12 of the first-stage Mach-Zehnder opticalinterferometer circuit 13 c. In addition, a light input side of thefirst optical waveguide 11 of the first-stage Mach-Zehnder opticalinterferometer circuit 13 c is connected to the cross port 19 of thelight input side circuit 1.

Further, an optical signal output from the cross port 19 among theoptical signals having a plurality of wavelength input from the lightinput terminal 17 of the first optical waveguide 11 of the light inputside circuit 1 is input from the first optical waveguide 11 of thefirst-stage Mach-Zehnder optical interferometer circuit 13 c of thesecond light output side circuit 3 and is then output from a lightoutput side (cross port 31) of the second optical waveguide 12 of thefinal-stage (in the present embodiment, the second stage) Mach-Zehnderoptical interferometer circuit 13 d.

Furthermore, the first optical waveguide 11 of the first light outputside circuit 2 is connected to the through port 18 of the light inputside circuit 1. The optical signal output from the through port 18 amongthe optical signals having a plurality of wavelengths input from thelight input terminal 17 of the first optical waveguide 11 of the lightinput side circuit 1 is output from its output side (the through port28) through the first optical waveguide 11 of the first light outputside circuit 2.

Therefore, in the wavelength multiplexing and demultiplexing circuithaving the Mach-Zehnder optical interferometer circuits 13, light inputto the first optical waveguide or the second optical waveguide and thenoutput therefrom is called ‘through propagating light’, and light inputto one optical waveguide and output from the other optical waveguide iscalled ‘cross propagating light’. For example, light input to the lightinput side of the first optical waveguide 11 and output from the lightoutput side of the first optical waveguide 11 is the through propagatinglight, and light input to the light input side of the first opticalwaveguide 11 and output from the light output side of the second opticalwaveguide 12 is the cross propagating light.

In the first point-symmetrically connected optical interferometercircuit 5, the transmittance T_(CR) of light output from the cross port19 (that is, cross propagating light) is obtained by the followingExpressions (1), (2), and (3):

T _(CR)=4C(1−C)  (1),

C=4K(1−K)cos²(ΔΨ/2)  (2), and

ΔΨ=(2π/λ)n _(eff) ·ΔL _(C)  (3)

where ΔL_(C) is a difference in length between optical paths of thephase parts 9 of the respective Mach-Zehnder optical interferometercircuits 13, K is the coupling efficiency of the directional coupler 6,n_(eff) is an equivalent refractive index of a core (optical waveguide),and λ is a wavelength.

For example, when optical signals in the wavelength bands of 1.31 μm,1.49 μm, and 1.55 μm are input from the light input terminal 17 of thefirst point-symmetrically connected optical interferometer circuit 5,optical signals in wavelength bands of 1.31 μm and 1.49 μm are outputfrom the through port 18, and an optical signal in a wavelength band of1.55 μm is output from the cross port 19. In addition, these wavelengthbands are currently used in the B-PON system as shown in FIG. 26.

In this case, for the central wavelengths of two of these threewavelengths, when C is zero, T_(CR) is zero. Therefore, thetransmittance of the optical signals output from the through port 18T_(CR) is 1−T_(CR)=1, and thus two specified wavelengths are output fromthe through port 18.

Here, for the two wavelengths, since C is zero, the relationship cos²(ΔΨ/2)=0 is established in Expression (2). For example, ΔL_(C) isdetermined to satisfy ΔΨ/2=(2m+3)π at the wavelength of 1.31 μm andΔΨ/2=(2m+1)π at the wavelength of 1.49 μm.

Next, at a wavelength of 1.55 μm, T_(CR) is 1, and the cross port 19 isbroadband. Therefore, C is set to about 0.5 for the wavelength of 1.55μm. The K value of the directional coupler 6 is calculated to satisfythe above-mentioned relationship.

Finally, since the wavelength band of 1.31 μm is the widest transmissionband, it is possible to widen a wavelength band by setting the value ofK to about zero or 1 (100%) at the wavelength of 1.31 μm, which is thecentral wavelength thereof.

For example, when manufacturing a quartz-based optical waveguide circuithaving a relative index difference Δ of 0.45% and a size of 7.5×7.5 μm,the value of ΔL_(C) calculated according to the above-mentioned processis 7.71 μm, and the coupling efficiency K of the directional coupler atthe wavelength of 1.31 μm is about zero or 1. In addition, the couplingefficiency K of the directional coupler at the wavelength of 1.55 μm isabout 0.77.

Any type of directional coupler 6 will be used if the above-mentionedconditions are satisfied. However, in general, the directional couplerhaving a short coupling part in length is manufactured with a low errorrate. Therefore, it is preferable that the length of the coupling partof the directional coupler 6 shown in FIG. 3 be short.

That is, for example, the length of the coupling part of the directionalcoupler 6 satisfying the above-mentioned conditions is 924 μm or 2184μm, which is a wavelength characteristic of the coupling efficiency ofthe directional coupler. As shown in FIGS. 4A and 4B, even when thelength of the coupling part of the directional coupler 6 is 924 μm (FIG.4A) or 2184 μm (FIG. 4B), the above-mentioned conditions are satisfied.In this case, it is preferable that the length of the coupling part ofthe directional coupler 6 be 924 μm since a decrease in manufacturingerrors is expected.

Further, according to the present embodiment, in the firstpoint-symmetrically connected optical interferometer circuit 5, thedirectional coupler 6 is formed such that the length of the couplingpart thereof is 924 μm. In addition, in the wavelength characteristicsshown in FIG. 4A, the coupling efficiency K of the directional coupler 6is set to about 1 at a wavelength of 1.31 μm, and a differentialcoefficient dK/dλ of the coupling efficiency K with respect to awavelength of 1.55 μm is set to satisfy dK/dλ<0.

From the above-mentioned viewpoint, the point-symmetrically connectedoptical interferometer circuit can exhibit a broadband wavelengthmultiplexing and demultiplexing function. Based on the above-mentionedexamination, the first point-symmetrically connected opticalinterferometer circuit 5 and the second point-symmetrically connectedoptical interferometer circuit 7 having the same functional structure asthe first point-symmetrically connected optical interferometer circuit5, both being used in the present embodiment, have a pass wavelengthband characteristic (light transmission characteristic) for throughpropagating light represented by a characteristic line a shown in FIG.5.

Further, as can be seen from the characteristic line a, for example, theloss of the through propagating light is low in a wavelength band of1.31 μm (1.26 to 1.36 μm) and a wavelength band of 1.49 μm (1.48 to 1.50μm). That is, the point-symmetrically connected optical interferometercircuits 5 and 7 each have at least one low through loss wavelength band(in other words, through propagating wavelength) where the loss of thethrough propagating light is low.

Furthermore, pass wavelength band characteristics (light transmissioncharacteristics) of cross propagating light for the first and secondpoint-symmetrically connected optical interferometer circuits 5 and 7are represented by a characteristic line b in FIG. 5. The first andsecond point-symmetrically connected optical interferometer circuits 5and 7 each include a wavelength band of 1.55 μm (1.55 μm to 1.56 μm) andhave at least one low through loss wavelength band (that is, crosspropagating wavelength) where the loss of the cross propagating light islow. In addition, these characteristic lines a and b are obtained bysimulation.

Therefore, when optical signals in a wavelength band of 1.31 μm, awavelength band of 1.49 μm, and a wavelength band of 1.55 μm are inputto the light input terminal (the light input terminal 17 for opticalsignals having a plurality of wavelengths) of the firstpoint-symmetrically connected optical interferometer circuit 5 (that is,the light input side circuit 1 in the present embodiment), the opticalsignals in the wavelength band of 1.31 μm and the wavelength band of1.49 μm can be output from the through port 18 with low loss. Also, theoptical signal in the wavelength band of 1.55 μm can be output from thecross port 19 with low loss.

Moreover, contrary to the above, when the optical signals in thewavelength band of 1.31 μm and the wavelength band of 1.49 μm are inputto the through port 18 and the optical signal in the wavelength band of1.55 μm is input to the cross port 19, the optical signals in thesewavelength bands are multiplexed through a course contrary to the above,and are then output from the light input terminal with low loss.

However, as can be seen from the characteristic lines a and b in FIG. 5,in the wavelength band of 1.31 (1.26 to 1.36 μm), the wavelength band of1.49 μm (1.48 to 1.50 μm), and the wavelength band of 1.55 μm (1.55 to1.56 μm), isolation is about 15 dB as seen from a band (the overallfrequency range of the band), which indicates that isolation is notsufficiently high.

Further, in the present embodiment, the first optical waveguide 11 ofthe second point-symmetrically connected optical interferometer circuit7 having the same functional structure as the first point-symmetricallyconnected optical interferometer circuit 5 is connected to the throughport 18 of the first point-symmetrically connected opticalinterferometer circuit 5 constituting the light input side circuit 1. Inthis way, the through propagating light output from the through port 18of the first point-symmetrically connected optical interferometercircuit 5 passes through the first optical waveguide 11 of the secondpoint-symmetrically connected optical interferometer circuit 7 and isthen output from a through port 28, which is an output terminal thereof.

The characteristic line a in FIG. 6 shows an example of the lighttransmission characteristics of light (through propagating light of thebroadband wavelength multiplexing and demultiplexing filter according tothe present embodiment) input to the light input terminal 17 of thelight input side circuit 1 and then output from the through port 28 ofthe first light output side circuit 2. In addition, a characteristicline a′ in FIG. 6 shows a light transmission characteristic of thethrough propagating light of the light input side circuit 1 composed ofonly the first point-symmetrically connected optical interferometercircuit 5. Further, the respective characteristic lines shown in FIG. 6are obtained by simulation.

As can be apparently seen from these characteristic lines a and a′, thefirst light output side circuit 2 composed of the secondpoint-symmetrically connected optical interferometer circuit 7 havingthe same functional structure as the first point-symmetrically connectedoptical interferometer circuit 5 is connected to the through port 18 ofthe light input side circuit 1 composed of the first point-symmetricallyconnected optical interferometer circuit 5, so that it is possible toincrease the loss of the through propagating light in a great losswavelength band (a wavelength band of 1.55 μm) while maintaining thebroadband wavelength demultiplexing function.

The wavelength band of 1.55 μm, which is the great loss wavelength bandof the through propagating light, is a pass wavelength band of the crosspropagating light of the broadband wavelength multiplexing anddemultiplexing filter in the present embodiment. That is, the connectionstructure between the first and second point-symmetrically connectedoptical interferometer circuits 5 and 7 increases the isolation of thethrough propagating light with respect to the pass wavelength band ofthe cross propagating light.

Further, the wavelength band of 1.55 μm, which is the pass wavelengthband of the cross propagating light, has a narrow band, but requires ahigh isolation characteristic in a wide wavelength band. Therefore, inthe present embodiment, the Mach-Zehnder optical interferometer circuits13 c and 13 d having different length structures from those of theMach-Zehnder optical interferometer circuits 13 a and 13 b constitutingthe first and second point-symmetrically connected opticalinterferometer circuits 5 and 7 are multistage-connected to form thesecond light output side circuit (the difference of the length structurecauses different multiplexing and demultiplexing characteristic andlight transmission characteristic). In addition, the second light outputside circuit is connected to the cross port 19 of the firstpoint-symmetrically connected optical interferometer circuit 5constituting the light input side circuit 1.

At least one of the Mach-Zehnder optical interferometer circuits 13 cand 13 d (in the present embodiment, both the Mach-Zehnder opticalinterferometer circuits 13 c and 13 d) is formed such that the loss ofthe cross propagating light thereof has a maximum value in at least oneof the low through loss wavelength bands of the firstpoint-symmetrically connected optical interferometer circuit 5.

Specifically, the first-stage Mach-Zehnder optical interferometercircuit 13 c has the light transmission characteristic of the crosspropagating light represented by a characteristic line b′ in FIG. 7.That is, the first-stage Mach-Zehnder optical interferometer circuit 13c has the maximum value of loss in a wavelength band of 1.49 μm (1.48 to1.50 μm), which is the low through loss wavelength band, and in awavelength band of 1.31 μm (1.26 to 1.36 μm). In addition, thecharacteristic lines shown in FIG. 7 are obtained by simulation.

Further, the second-stage Mach-Zehnder optical interferometer circuit 13d has the light transmission characteristic of the cross propagatinglight represented by a characteristic line b″ in FIG. 7. That is, thesecond-stage Mach-Zehnder optical interferometer circuit 13 d has themaximum value of loss in a wavelength band of 1.31 μm (1.26 to 1.36 μm),which is one of the low through loss wavelength bands, and a loss valueis large in the vicinity of this maximum value.

Furthermore, as indicated by a characteristic line B of FIG. 7, thefirst point-symmetrically connected optical interferometer circuit 5 hasat least one low cross loss wavelength band where the loss of the crosspropagating light is low as described above. The vicinity of awavelength of 1.27 μm where the loss of the cross propagating light ofthe Mach-Zehnder optical interferometer circuit 13 d is maximum iswithin one of the low through loss wavelength bands of thepoint-symmetrically connected optical interferometer circuit 5. That is,the Mach-Zehnder optical interferometer circuit 13 d is constructed suchthat the loss of the cross propagating light thereof has the maximumvalue in at least one of the low through loss wavelength bands of thepoint-symmetrically connected optical interferometer circuit 5.

The following effects can be obtained by providing these Mach-Zehnderoptical interferometer circuits 13 c and 13 d to form the second lightoutput side circuit 3. That is, the cross propagating light componentsof the Mach-Zehnder optical interferometer circuits 13 c and 13 d havethe maximum values of loss in a wavelength band of 1.49 μm. Therefore,as represented by the characteristic line b of FIG. 7, in the broadbandwavelength multiplexing and demultiplexing filter of the presentembodiment, the loss of the cross propagating light exceeds 25 dB in thewavelength band of 1.49 μm. That is, the broadband wavelengthmultiplexing and demultiplexing filter of the present embodiment canhave a high isolation characteristic of the cross propagating light withrespect to the through propagating light in the wavelength of 1.49 μm.

Further, since the loss of the cross propagating light of the firstpoint-symmetrically connected optical interferometer circuit 5 is low ina wavelength band of 1.31 μm, it is difficult to increase the isolationof the cross propagating light to the through propagating light in thewavelength band of 1.31 μm with only the first point-symmetricallyconnected optical interferometer circuit 5. On the other side, thepresent embodiment makes it possible to increase the isolation of thecross propagating light to the through propagating light in thewavelength band of 1.31 μm by connecting the Mach-Zehnder opticalinterferometer circuits 13 c and 13 d to the cross port of the firstpoint-symmetrically connected optical interferometer circuit 5.

In other words, both the cross propagating light components of theMach-Zehnder optical interferometer circuits 13 c and 13 d have themaximum values of loss in the wavelength band of 1.31 μm. Particularly,the second-stage Mach-Zehnder optical interferometer circuit 13 d hasthe maximum value of loss around a wavelength of 1.27 μm where the crossloss of the point-symmetrically connected optical interferometer circuit5 is low. In this way, the loss of the cross propagating light of thebroadband wavelength multiplexing and demultiplexing filter according tothe present embodiment can exceed 25 dB in the wavelength band of 1.31μm.

Example 1

According to an example of the first embodiment, the following broadbandwavelength multiplexing and demultiplexing filter is manufactured. Thatis, first, an under clad film made of quartz-based glass and aTiO₂-doped core film are formed on a silicon substrate using a flamehydrolysis depositing method. At that time, the relative indexdifference Δ of the core to the clad is set to 0.4%, and the filmthickness of the core is set to 7.5 μm.

Subsequently, an optical circuit pattern is transferred onto the coreusing a photomask on which a circuit of the broadband wavelengthmultiplexing and demultiplexing filter shown in FIG. 1 is drawn by aphotolithography method and a reactive ion etching method. Then, an overclad film made of quartz-based glass is formed using the flamehydrolysis depositing method again. In this way, the broadbandwavelength multiplexing and demultiplexing filter formed by the opticalwaveguide of the core and having a circuit structure shown in FIG. 1 ismanufactured.

Further, the width of the core constituting the optical waveguide is setto 7.5 μm. In the point-symmetrically connected optical interferometercircuits 5 and 7, ΔL_(C) of the directional coupler is set to 7.71 μm,the pitch between the directional couplers 6 (as shown in FIG. 3, thedistance between the center of the first optical waveguide 11 and thecenter of the second optical waveguide 12) is set to 11.1 μm, and thelength of the coupling part of the directional coupler is set to 924 μm.

Furthermore, in the Mach-Zehnder optical interferometer circuit 13 c,ΔL_(C) of the directional coupler is set to 11.82 μm, the pitch betweenthe directional couplers 6 (as shown in FIG. 3, the distance between thecenter of the first optical waveguide 11 and the center of the secondoptical waveguide 12) is set to 11.1 μm, and the length of the couplingpart of the directional coupler is set to 167 μm.

Moreover, in the Mach-Zehnder optical interferometer circuit 13 d,ΔL_(C) of the directional coupler is set to 2.18 μm, the pitch betweenthe directional couplers 6 (as shown in FIG. 3, the distance between thecenter of the first optical waveguide 11 and the center of the secondoptical waveguide 12) is set to 11.1 μm, and the length of the couplingpart of the directional coupler is set to 167 μm.

The characteristics of the broadband wavelength multiplexing anddemultiplexing filter according to the example 1 are represented by thecharacteristic lines a and b in FIG. 8. In addition, the characteristicline a is an actual measurement of the characteristic of the throughpropagating light, that is, the characteristic of light input to thelight input terminal 17 and then output from the through port 28. Thecharacteristic line b is an actual measurement of the characteristic ofthe cross propagating light, that is, the characteristic of light inputto the light input terminal 17 and then output from the cross port 31.

As can be apparently seen from these characteristic lines a and b, aninsertion loss is about 2.5 dB in a wavelength of 1.31 μm (1.26 μm to1.36 μm), and isolation is greater than 27 dB. In addition, theinsertion loss is about 1.5 dB in a wavelength of 1.49 μm (1.48 μm to1.50 μm), and isolation is greater than 25 dB. The insertion loss isabout 1.5 dB in a wavelength of 1.55 μm (1.55 μm to 1.56 μm), andisolation is greater than 25 dB.

As can be seen from the results, according to the example 1,characteristics substantially corresponding to design values areobtained, and thus the validity of design is established.

Second Embodiment

Next, a second embodiment of the broadband wavelength multiplexing anddemultiplexing filter according to the present invention will bedescribed. The second embodiment has the circuit structure shown in FIG.9, and has substantially the same structure as that in the firstembodiment. The second embodiment differs from the first embodiment inthat a third point-symmetrically connected optical interferometercircuit 8 having the same functional structure as that of the firstpoint-symmetrically connected optical interferometer circuit 5 isprovided instead of the first-stage Mach-Zehnder optical interferometercircuit 13 c constituting the second light output side circuit 3.

Further, the Mach-Zehnder optical interferometer circuit 13 d is formedsuch that the loss of the cross propagating light thereof has a maximumvalue in the wavelength band of 1.31 μm.

Example 2

An example 2 of the second embodiment forms a broadband wavelengthmultiplexing and demultiplexing filter in the same manner as that in theexample 1 of the first embodiment. In the example 2, the relative indexdifference Δ of a core to a clad is set to 0.3%, and the film thicknessof the core is set to 8.0 μm. Then, an optical circuit pattern of thecore is formed by transferring the pattern shown in FIG. 9 thereto.

Further, the width of the core constituting the optical waveguidecircuit is set to 8.0 μm. In the point-symmetrically connected opticalinterferometer circuits 5, 7, and 8, Δ_(LC) of the directional coupleris set to 10.81 μm, the pitch between the directional couplers 6 is setto 11.6 μm, and the length of the coupling part of the directionalcoupler is set to 810 μm.

Furthermore, in the Mach-Zehnder optical interferometer circuit 13 d, Δ_(LC) of the directional coupler is set to 11.82 μm, the pitch betweenthe directional couplers 6 is set to 12.0 μm, and the length of thecoupling part of the directional coupler is set to 30 μm.

The characteristics of the broadband wavelength multiplexing anddemultiplexing filter according to the example 2 are represented bycharacteristic lines a and b in FIG. 10. The characteristic line a is anactual measurement of the characteristic of the through propagatinglight, that is, the characteristic of light input to the light inputterminal 17 and then output from the through port 28. The characteristicline b is an actual measurement of the characteristic of the crosspropagating light, that is, the characteristic of light input to thelight input terminal 17 and then output from the cross port 31.

As can be apparently seen from these characteristic lines a and b, aninsertion loss is about 1.5 dB in a wavelength of 1.31 μm (1.26 μm to1.36 μm), and isolation is greater than 40 dB. In addition, theinsertion loss is about 1.5 dB in a wavelength of 1.49 μm (1.48 μm to1.50 μm), and isolation is greater than 30 dB. The insertion loss isabout 1.5 dB in a wavelength of 1.55 μm (1.55 μm to 1.56 μm), andisolation is greater than 30 dB. Therefore, the example 2 also hascharacteristics corresponding to design values, and thus the validity ofdesign is established.

As described above, according to the first and second embodiments, theoptical waveguide circuit formed on a substrate has the firstpoint-symmetrically connected optical interferometer circuit 5 formed byaccurately point-symmetrically connecting two equal Mach-Zehnder opticalinterferometer circuits 13 a and 13 b in series. The point-symmetricallyconnected optical interferometer circuit 5 has a characteristic ofmultiplexing and demultiplexing a broadband wavelength.

A light input side circuit 1 is formed by connecting one or morepoint-symmetrically connected optical interferometer circuits 5 inseries, and the light input terminal 17 of the first optical waveguide11 of the light input side circuit 1 is composed of an input port forthe optical signals having a plurality of wavelengths. Therefore, theoptical signals having a plurality of broadband wavelengths are reliablydemultiplexed by the light input side circuit 1 and then arerespectively output from the output side of the first optical waveguide11 and the output side of the second optical waveguide 12 constitutingthe light input side circuit 1.

The present inventors have proved the fact by examination, such assimulation, that, when a plurality of point-symmetrically connectedoptical interferometer circuit are connected to each other in series,the isolation characteristic of the pass wavelength band (transmissionwavelength band) of through propagating light (light input to the firstoptical waveguide 11 or the second optical waveguide 12 and then outputtherefrom) with respect to the pass wavelength band (transmissionwavelength band) of cross propagating light (light input to the firstoptical waveguide 11 and the second optical waveguide 12 and then outputfrom the other waveguide) is improved.

Further, the output terminal of the first optical waveguide 11 of thelight input side circuit 1 is composed of a through port, and thethrough port is connected to the first light output side circuit 2formed by connecting in series one or more point-symmetrically connectedoptical interferometer circuits 7 having the same functional structureas that of the point-symmetrically connected optical interferometercircuit 5. In this way, it is possible to increase the isolation of thethrough propagating light with respect to the pass wavelength band ofthe cross propagating light while maintaining the broadband wavelengthdemultiplexing characteristic.

Furthermore, according to the second embodiment, the output terminal ofthe second optical waveguide 12 of the light input side circuit 1 iscomposed of a cross port, and the cross port is connected to the secondlight output side circuit 3 including one or more Mach-Zehnder opticalinterferometer circuits 13 c and 13 d having a different lengthstructure from the Mach-Zehnder optical interferometer circuits 13 a and13 b constituting the point-symmetrically connected opticalinterferometer circuit 5. In this way, the characteristics caused by thestructures of the Mach-Zehnder optical interferometer circuits 13 c and13 d constituting the second light output side circuit 3 make itpossible to increase the isolation of cross propagating light to thepass wavelength band of through propagating light.

Moreover, all the light input side circuit 1 and the first and secondlight output side circuits 2 and 3 can multiplex broadband wavelengths,contrary to the demultiplexing, by changing the input direction of lightin the reverse direction.

Further, according to the present embodiment, it is possible tomultiplex and demultiplex optical signals having broadband wavelengthswith low loss, and thus it is possible to realize a broadband wavelengthmultiplexing and demultiplexing filter having an excellenthigh-isolation characteristic. Therefore, it is possible to manufacturea system at low costs, capable of simultaneously providing multi-channelimage signals with one optical fiber.

Furthermore, according to the first embodiment of the present invention,the second light output side circuit 3 has n-stage (where n is aintegral number equal to or greater than 2) Mach-Zehnder opticalinterferometer circuits 13 c and 13 d. As such, it is possible toreliably form a broadband wavelength multiplexing and demultiplexingfilter having the above effects by connecting the n-stage circuits toeach other to form the second light output side circuit 3.

Moreover, as a method for connecting the n-stage Mach-Zehnder opticalinterferometer circuits 13 c and 13 d, a connecting method is adopted inwhich cross propagating light travels from the first optical waveguide11 to the second optical waveguide 12 located at the next stage thereof.In this way, light output from the cross port of the light input sidecircuit 1 is input to the first optical waveguide 11, which is the firststage circuit of the second light output side circuit 3, and is thenoutput from the light output side of the second optical waveguide 12,which is the last stage circuit. Therefore, it is possible to repeatedlyperform an operation one or more times in which light is input from thefirst optical waveguide 11 of the Mach-Zehnder optical interferometercircuit and is then output to the second optical waveguide 12 (crosspropagation) in the course of the propagation of light through thesecond light output side circuit 3.

Further, it is possible to more reliably increase the isolation of thecross propagating light with respect to the through propagating light ofthe light input side circuit by setting the pass wavelength bandcharacteristic of the cross propagating light by the structures of theMach-Zehnder optical interferometer circuits 13 c and 13 d constitutingthe second light output side circuit 3.

Furthermore, in the present embodiment, the point-symmetricallyconnected optical interferometer circuit 5 has one or more low throughloss wavelength bands where the loss of through propagating light islow. In addition, the Mach-Zehnder optical interferometer circuits 13 cand 13 d (the Mach-Zehnder optical interferometer circuit 13 d in thesecond embodiment) constituting the second light output side circuit 3has a maximum value of cross propagating light loss in at least one ofthe plurality of low through loss wavelength bands. According to thisconfiguration, it is possible to increase the loss of the crosspropagating light in the low through loss wavelength band by thestructures of the Mach-Zehnder optical interferometer circuits 13 c and13 d (the Mach-Zehnder optical interferometer circuit 13 d in the secondembodiment) constituting the second light output side circuit 3.Therefore, it is possible to more reliably increase the isolationcharacteristic of cross propagating light with respect to the passwavelength band of through propagating light of the broadband wavelengthmultiplexing and demultiplexing filter.

Moreover, in the present embodiment, the point-symmetrically connectedoptical interferometer circuit 5 has one or more low cross losswavelength bands where the loss of cross propagating light is low. Inaddition, at least one of the Mach-Zehnder optical interferometercircuits 13 c and 13 d (the Mach-Zehnder optical interferometer circuit13 d in the second embodiment) constituting the second light output sidecircuit 3 has a maximum value of cross propagating light loss in atleast one of the one or more low through loss wavelength bands.According to this configuration, it is possible to increase the loss ofthe point-symmetrically connected optical interferometer circuit 5 inthe low through loss wavelength band by the Mach-Zehnder opticalinterferometer circuits 13 c and 13 d (the Mach-Zehnder opticalinterferometer circuit 13 d in the second embodiment). Therefore, it ispossible to more reliably increase the isolation characteristic of crosspropagating light with respect to the pass wavelength band of throughpropagating light of the broadband wavelength multiplexing anddemultiplexing filter.

Third Embodiment

Next, a third embodiment of a broadband wavelength multiplexing anddemultiplexing filter according to the present invention will bedescribed. A broadband wavelength multiplexing and demultiplexing filter50 of the third embodiment is constructed by forming the opticalwaveguide circuit shown in FIG. 11 on a substrate 15, and this circuitis constructed by accurately point-symmetrically connectingphase-part-intervention-type point-symmetrically connected opticalinterferometer circuits 52 a and 52 b in series.

In each of the phase-part-intervention-type point-symmetricallyconnected optical interferometer circuits 52 a and 52 b, opticalcouplers 53 a and 53 b are accurately point-symmetrically formed andhave the same structure, and a phase portion 54 is positioned betweenthe optical couplers 53 a and 53 b. In addition, the optical couplers 53a and 53 b each comprises a directional coupler 56 (see FIG. 3) and asecond phase part 58 having ΔL₀. Further, the length (phase amount) ΔL₀of the second phase part 58 is different from the length (phase amount)of a phase part 54.

In the phase-part-intervention-type point-symmetrically connectedoptical interferometer circuits 52 a and 52 b shown in FIG. 11, thearrangement pitches between the directional couplers 56 are equal toeach other. In the phase part 54 of the phase-part-intervention-typepoint-symmetrically connected optical interferometer circuit 52 a (whichis located at the left side of FIG. 11), the length of the first opticalwaveguide 11 is longer than that of the second optical waveguide 12 by aset length (in the present phase-part-intervention-typepoint-symmetrically connected optical interferometer circuit 52 b (whichis located at the right side of FIG. 11), the length of the secondoptical waveguide 12 is longer than that of the first optical waveguide11 by the set length (in the present embodiment, ΔL₁).

Optical couplers 53 a and 53 b constituting thephase-part-intervention-type point-symmetrically connected opticalinterferometer circuits 52 a and 52 b each have two second phase parts58 respectively formed in the first optical waveguide 11 and the secondoptical waveguide 12. Therefore, in each of thephase-part-intervention-type point-symmetrically connected opticalinterferometer circuits 52 a and 52 b, the difference between the lengthof the first optical waveguide 11 and the length of the second opticalwaveguide 12 becomes the set length (ΔL₁) since the set lengths ΔL₀ ofthe second phase parts 58 are canceled.

The above-mentioned set lengths ΔL₀ and ΔL₁ are properly set accordingto the circuit design of the respective phase-part-intervention-typepoint-symmetrically connected optical interferometer circuits 52 a and52 b.

As shown in FIG. 11, the light input terminal 17 of the first opticalwaveguide 11 is composed of an input port for optical signals having aplurality of wavelengths. An output terminal of the first opticalwaveguide 11 corresponding to the input port (an IN port) is composed ofthe through port 18, and an output terminal of the second opticalwaveguide 12 not provided with an input port is composed of the crossport 19.

In the phase-part-intervention-type point-symmetrically connectedoptical interferometer circuits 52 a and 52 b, the transmittance T_(CR)of light (that is, cross propagating optical signal) output from thecross port 19 is calculated by the following Expressions (1), (2), (3′),(4), and (5):

T _(CR)=4C(1−C)  (1),

C=4K(1−K)cos²(ΔΨ/2)  (2),

ΔΨ=n_(eff) ·ΔL ₁(2π/λ)  (3′)

K=4P(1−P)cos²(ΔΦ/2)  (4), and

ΔΦ=n _(eff) ·ΔL ₀(2π/λ)  (5)

where P is the coupling efficiency of the directional coupler 56, ΔL₀ isa difference in length between the optical paths of optical couplers,ΔL₁ is a difference in length between the optical paths ofphase-part-intervention-type point-symmetrically connected opticalinterferometer circuits, n_(eff) is an equivalent refractive index of acore (optical waveguide), and λ is a wavelength.

Example 3

As an example 3 of the third embodiment, the broadband wavelengthmultiplexing and demultiplexing filter 50 is designed as follows. Acircuit design using Expressions (1) to (5) will be described below.Further, in the circuit design, optical signals in the wavelength bandsof 1.31 μm, 1.49 μm, and 1.55 μm are output from the through port 18,and an optical signal having a wavelength band of 1.65 μm is output fromthe cross port 19.

For two wavelength bands of 1.31 μm and 1.55 μm having the greatestwavelength difference therebetween, since C is almost zero and T_(CR) isalmost zero, the transmittance of light propagated to the through port18 is about 1 from the relationship 1−T_(CR)≅1. Therefore, the twospecified wavelengths are output from the through port 18. Since C isalmost zero, cos² (ΔΨ/2) is almost zero in Expression (2). That is, ΔL₁is determined that the relationship ΔΨT/2≅(2m+3)π is satisfied in thewavelength band of 1.31 μm, and that the relationship ΔΨ/2≅(2m+1)π issatisfied in the wavelength band of 1.55 μm.

Next, when T_(CR) is almost one in the wavelength band of 1.65 μm, Cisabout 0.5. The coupling efficiency K of the optical coupler isdetermined by calculation so as to satisfy the above condition.

Subsequently, in order to widen the wavelength band of 1.31 μm, thewavelength band of 1.49 μm, and the wavelength band of 1.55 μm, it isnecessary to extend a wavelength band where C is almost zero in thevicinities of these wavelength bands as widely as possible. That is, awavelength band where K is almost zero is extended as widely aspossible. In addition, at the same time, in a wavelength band of 1.65μm, the coupling efficiency P of the directional coupler and ΔL₀ arecalculated so as to obtain the above-mentioned value of K.

In this way, the directional coupling efficiency P, the set length ΔL₀of the second phase part 58, and the set length ΔL₁ of the phase part 54are calculated, and the broadband wavelength multiplexing anddemultiplexing filter 50 is manufactured in a state in which therelative index difference Δ of the core with respect to the clad is0.8%. Then, an optical coupling characteristic and a filtercharacteristic (a wavelength characteristic of transmittance) of thefilter are shown in FIGS. 12A and 12B.

As can be seen from FIG. 12B, the broadband wavelength multiplexing anddemultiplexing filter 50 can output optical signals in the wavelengthbands of 1.31 μm, 1.49 μm, and 1.55 μm from the through port 18 andoutput an optical signal in the wavelength band of 1.65 μm from thecross port 19.

Fourth Embodiment

Next, a fourth embodiment of the broadband wavelength multiplexing anddemultiplexing filter according to the present invention will bedescribed. The fourth embodiment has a circuit structure shown in FIG.13 and is different from the third embodiment in that an additionalbroadband wavelength multiplexing and demultiplexing filter is connectedin series to the through port 18 of the circuit shown in FIG. 11, and inthat an additional broadband wavelength multiplexing and demultiplexingfilter and two (a two-stage structure) filter circuits are connected inseries to the cross port of the circuit shown in FIG. 11. The fourthembodiment will be described below in detail with reference to FIG. 13.

A broadband wavelength multiplexing and demultiplexing filter 70 shownin FIG. 13 comprises a first-stage broadband wavelength multiplexing anddemultiplexing filter 72, second-stage broadband wavelength multiplexingand demultiplexing filters 74 a and 74 b respectively connected inseries to the through port 18 (a port shown on the upper side of FIG.13) and the cross port 19 (a port shown on the lower side of FIG. 13) ofthe first-stage broadband wavelength multiplexing and demultiplexingfilter 72, and filter circuits 76 and 78 having a two-stage structurethat are connected in series to a cross port of the second-stagebroadband wavelength multiplexing and demultiplexing filter 74 b. Inaddition, the second-stage broadband wavelength multiplexing anddemultiplexing filters 74 a and 74 b have the same structure as thefirst-stage broadband wavelength multiplexing and demultiplexing filter72. Since the first-stage broadband wavelength multiplexing anddemultiplexing filter 72 has already been described in detail in theexample 3, a description thereof will be omitted.

The filter circuits 76 and 78 are connected in series to a seconddirectional coupler 80 and a third phase part 82, respectively. Inaddition, the second directional coupler 80 is different from thedirectional coupler 56 (see FIG. 11) in the first-stage or second-stagebroadband wavelength multiplexing and demultiplexing filter 72, 74 a, or74 b in coupling efficiency. Further, the third phase part 82 isdifferent from the second phase part 58 (see FIG. 11. ΔL₀ in FIG. 11) inthe first-stage or second-stage broadband wavelength multiplexing anddemultiplexing filter 72, 74 a, or 74 b in length (phase amount).

In the broadband wavelength multiplexing and demultiplexing filter 70,optical signals input to the input port and then output from a throughport 84 (the lower port of the second-stage broadband wavelengthmultiplexing and demultiplexing filter 74 a in FIG. 13) pass throughonly the optical waveguide located at the through port side of thefirst-stage and second-stage broadband wavelength multiplexing anddemultiplexing filters 72 and 74 a.

Further, optical signals input to the input port and then output from across port 86 (the upper port of the second-stage broadband wavelengthmultiplexing and demultiplexing filter circuit 78 in FIG. 13) passthrough only the optical waveguide located at the cross port side of thefirst-stage and second-stage broadband wavelength multiplexing anddemultiplexing filters 72 and 74 b, and the first-stage and second-stagefilter circuits 76 and 78.

Example 4

According to an example 4 of the fourth embodiment, the broadbandwavelength multiplexing and demultiplexing filter 70 shown in FIG. 13 ismanufactured. FIG. 14 is a graph showing the design of wavelengthcharacteristics of transmittance when optical signals pass through thefirst-stage and second-stage broadband wavelength multiplexing anddemultiplexing filters 72 and 74 a located at the through port, and FIG.15 is a graph showing the design of the wavelength characteristic oftransmittance when optical signals pass through the first-stage andsecond-stage broadband wavelength multiplexing and demultiplexingfilters 72 and 74 b and the first-stage and second-stage filter circuits76 and 78 located at the cross port. FIG. 16 is a graph showing theexamination result of the wavelength characteristic (light transmissioncharacteristic) of transmittance of the broadband wavelengthmultiplexing and demultiplexing filter 70 formed on the basis of FIGS.14 and 15.

In FIG. 15, the wavelength characteristic of the transmittance of anoptical signal output from the cross port 86 of the second-stagebroadband wavelength multiplexing and demultiplexing filter 74 b has avalley-shaped (transmittance is lowered, and thus a large loss occurs)spectrum in a wavelength band of 1.31 μm. However, it is understood thatisolation in the wavelength band of 1.31 μm can be improved larger than40 dB by forming the (first-stage) filter circuit 76 having thewavelength characteristic of the transmittance of an optical signal inwhich a spectrum in the wavelength band of 1.31 μm is a valley shape andbypassing the optical signal therethrough.

In FIG. 15, the wavelength characteristic of the transmittance of anoptical signal output from the cross port of the second-stage broadbandwavelength multiplexing filter 74 b becomes a mountain-shaped (a portionwhere the transmittance increases and a loss is low) spectrum in awavelength band of 1.43 μm in order to improve isolation in thewavelength bands of around 1.49 μm, 1.55 μm and 1.43 μm. However, it isunderstood that isolation in the wavelength band of 1.43 μm can beimproved larger than 30 dB by forming the (second-stage) filter circuit78 having the wavelength characteristic of the transmittance of anoptical signal in which a spectrum in the wavelength band of 1.43 μm isa valley shape and by passing the optical signal therethrough.

As such, in the example 4, when the wavelength characteristic of thetransmittance of the optical signal output from the cross port of thesecond-stage broadband wavelength multiplexing and demultiplexing filter74 b has a valley-shaped spectrum or a mountain-shaped spectrum, it ispossible to construct the broadband wavelength multiplexing anddemultiplexing filter 70 having a broad band and high isolation bymaking a wavelength band having a peak value of each mountain-shapedspectrum or a wavelength band having a peak value of each valley-shapedspectrum equal to a wavelength band having a peak value of thevalley-shaped spectrum of the wavelength characteristic of thetransmittance of the first-stage and second-stage filter circuits 76 and78 arranged at the latter stage.

The broadband wavelength multiplexing and demultiplexing filter 70having a circuit structure shown in FIG. 13 is formed in considerationof FIGS. 14 and 15. The examination result thereof is shown in FIG. 16.In the broadband wavelength multiplexing and demultiplexing filter 70,first, an under clad film and a core film made of quartz-based glassobtained by doping GeO₂ are formed on a silicon substrate using a flamehydrolysis depositing method. At that time, the relative indexdifference Δ is set to 0.8%, and the film thickness of the core is setto 6.5 μm. Subsequently, an optical circuit pattern is transferred tothe core using a photo mask on which a circuit of the broadbandwavelength multiplexing and demultiplexing filter 70 shown in FIG. 13 isdrawn by a photolithography method and a reactive ion etching method.Then, an over clad film made of quartz-based glass is formed using theflame hydrolysis depositing method again, thereby manufacturing thebroadband wavelength multiplexing and demultiplexing filter 70.

Further, the design values of the respective optical circuits are asfollows. In the first-stage and second-stage broadband wavelengthmultiplexing and demultiplexing filters 72, 74 a, and 74 b, ΔL₀ of thesecond phase part 58 is 8.03 μm, ΔL₁ of the phase part 54 is 6.72 μm,the pitch between the directional couplers 56 is 9.4 μm, and the lengthof the coupling part of the directional coupler 56 is 0 μm. In thefirst-stage filter circuit 76, ΔL_(C) of the third phase part 82 is 2.25μm, the pitch between the second directional couplers 80 is 9.4 μm, andthe length of the coupling part of the second directional coupler 80 is427 μm. In the second-stage filter circuit 78, Δ_(LC) of the third phasepart 82 is 3.43 μm, the pitch between the second directional couplers 80is 9.4 μm, and the length of the coupling part of the second directionalcoupler 80 is 427 μm.

As can be seen from the graph of the wavelength characteristic oftransmittance shown in FIG. 16, in all wavelength bands of 1.31 μm (1.26to 1.36 μm), 1.49 μm (1.48 to 1.50 μm), 1.55 μm (1.55 to 1.56 μm), and1.65 μm (1.635 to 1.670 μm), an insertion loss is equal to or less than2.5 dB, and isolation is substantially greater than 40 dB.

Fifth Embodiment

Next, a fifth embodiment of the broadband wavelength multiplexing anddemultiplexing filter according to the present invention will bedescribed. The fifth embodiment has a circuit structure shown in FIG. 17and is different from the fourth embodiment shown in FIG. 13 in that afilter circuit having one-stage structure is used.

Example 5

According to an example 5 of the fifth embodiment, a broadbandwavelength multiplexing and demultiplexing filter 90 is formed in thesame manner as the example 4. That is, first, a core film made ofquartz-based glass obtained by doping GeO₂ and an under clad film areformed on a silicon substrate using a flame hydrolysis depositingmethod.

At that time, the relative index difference Δ is set to 0.8%, and thefilm thickness of the core is set to 6.5 μm. Subsequently, an opticalcircuit pattern is transferred to the core through a photo mask on whichthe broadband wavelength multiplexing and demultiplexing filter 90 shownin FIG. 17 having a circuit structure in which a wavelength band of 1.31μm and a wavelength band of 1.49 μm are output from the through port andin which a wavelength band of 1.55 μm is output from the cross port isdrawn by a photolithography method and a reactive ion etching method.Then, an over clad film made of quartz-based glass is formed using theflame hydrolysis depositing method again, thereby manufacturing thebroadband wavelength multiplexing and demultiplexing filter 90.

Further, the design values of the respective optical circuits are asfollows. In the first-stage and second-stage broadband wavelengthmultiplexing and demultiplexing filters 72, 74 a, and 74 b, ΔL₀ of thesecond phase part 58 is 3.15 μm, ΔL₁ of the phase part 54 is 10.77 μm,the pitch between the directional couplers 56 is 11.2 μm, and the lengthof the coupling part of the directional coupler 56 is 423 μm. In thefirst-stage filter circuit 76, ΔL_(C) of the third phase part 82 is11.79 μm, the pitch between the second directional couplers 80 is 9.4μm, and the length of the coupling part of the second directionalcoupler 80 is 566 μm.

FIG. 18 shows the wavelength characteristic of the transmittance of thebroadband wavelength multiplexing and demultiplexing filter in FIG. 17manufactured by the above-mentioned design. As can be seen from thegraph shown in FIG. 18, in all wavelength bands of 1.31 μm (1.26 to 1.36μm), 1.49 μm (1.48 to 1.50 μm), and 1.55 μm (1.55 to 1.56 μm), aninsertion loss is equal to or less than 2.0 dB, and isolation issubstantially greater than 40 dB.

Sixth Embodiment

Next, a sixth embodiment of the broadband wavelength multiplexing anddemultiplexing filter according to the present invention will bedescribed.

A circuit using the broadband wavelength multiplexing and demultiplexingfilter according to the sixth embodiment is a broadband opticalsplitter/router 100 having a circuit structure shown in FIG. 19. Thebroadband optical splitter/router 100 is manufactured by transferring anoptical circuit pattern on the core through a photo mask having acircuit structure thereon in which the broadband wavelength multiplexingand demultiplexing filters 50 and 70 shown in FIGS. 11 and 13, an arraywaveguide lattice 102, and an eight-branch optical splitter 104 areformed on a substrate 106 as a monolithic circuit, using aphotolithography method and a reactive ion etching method, and byforming an over clad film made of quartz-based glass thereon by usingthe flame hydrolysis depositing method again.

In the broadband optical splitter/router 100 shown in FIG. 19, first,the input WDM optical signals in the wavelength bands of 1.31 μm, 1.49μm, 1.55 μm, and 1.65 μm are demultiplexed by the broadband wavelengthmultiplexing and demultiplexing filter 70. Then, optical signals in thewavelength bands of 1.31 μm, 1.49 μm, and 1.55 μm are split into eightoptical signals having the same intensity by the eight-branch opticalsplitter 104. On the other hand, the optical signal in the wavelengthband of 1.65 μm is demultiplexed into eight channels having a wavelengthgap of 5 nm by the array waveguide lattice 102, functioning as anoptical router. Then, the split optical signals in the wavelength bandsof 1.31 μm, 1.49 μm, and 1.55 μm and the split optical signal in thewavelength band of 1.65 μm are multiplexed by the broadband wavelengthmultiplexing and demultiplexing filter 50 to be output. That is, thebroadband wavelength multiplexing and demultiplexing filter 70 shown inFIG. 13 is used for demultiplexing, and the broadband wavelengthmultiplexing and demultiplexing filter 50 shown in FIG. 11 is used formultiplexing.

Example 6

According to an example 6 of the sixth embodiment, the broadband opticalsplitter/router 100 shown in FIG. 19 is manufactured in the same manneras that in the above-mentioned examples. In addition, the design valuesof the respective optical circuits are as follows. In the broadbandwavelength multiplexing and demultiplexing filter 70 for demultiplexing,ΔL₀ of the second phase part is 8.03 μm, ΔL₁ of the phase part is 6.72μm, the pitch between the directional couplers is 11.2 μm, and thelength of the coupling part of the directional coupler is 270 μm. In thefirst-stage filter circuit 76, Δ_(LC) of the third phase part is 2.25μm, the pitch between the second directional couplers is 9.4 μm, and thelength of the coupling part of the second directional coupler is 427 μm.In the second-stage filter circuit 78, Δ_(LC) of the third phase part is2.25 μm, the pitch between the directional couplers is 9.4 μm, and thelength of the coupling part of the directional coupler is 427 μm. In thebroadband wavelength multiplexing and demultiplexing filter 50 formultiplexing, ΔL₀ of the second phase part is 8.03 μm, ΔL₁ of the phasepart is 6.72 μm, the pitch between the directional couplers is 11.2 μm,and the length of the coupling part of the directional coupler is 270μm.

The broadband optical splitter/router 100 is manufactured according tothe above-mentioned design, and the wavelength characteristic of thetransmittance of the optical signal output from each channel ch is shownin FIGS. 20A and 20B. As can be seen from FIGS. 20A and 20B, in allwavelength bands of 1.31 μm (1.26 to 1.36 μm), 1.49 μm (1.48 to 1.50μm), and 1.55 μm (1.55 to 1.56 μm), an insertion loss is equal to orless than 12.7 dB. On the other side, in the wavelength band of 1.65 μm(1.635 μm to 1.670 μm), the insertion loss is equal to or less than 7.5dB, and isolation is substantially greater than 30 dB. As a result, itis possible to realize an optical splitter function in the wavelengthbands of 1.31 μm, 1.49 μm, and 1.55 μm, and it is possible to realize anoptical router function in the wavelength band of 1.65 μm. Thus, it ispossible to realize a high-function device by monolithic-integratinganother waveguide circuit part.

Seventh Embodiment

Next, a circuit of a seventh embodiment using the broadband wavelengthmultiplexing and demultiplexing filter according to the presentinvention will be described. The circuit of the seventh embodiment is anoptical splitter 120 with a broadband wavelength multiplexing anddemultiplexing filter function, which has a circuit structure shown inFIG. 21.

In the manufacture of the optical splitter 120, first, a core film madeof quartz-based glass in which TiO₂ is doped is formed on a quartz glasssubstrate by a flame hydrolysis depositing method. At that time, therelative index difference Δ is set to 0.4%, and the film thickness ofthe core is set to 7.0 μm. Subsequently, an optical circuit patterncomprising a circuit pattern of an eight-branch optical splitter 122 forsplitting the input optical signal in the wavelength band of 1.55 μminto eight optical signals having the same intensity and a circuitpattern of the broadband wavelength multiplexing and demultiplexingfilter 50 is transferred to the core through a photo mask in which amonolithic-integrated circuit structure is drawn on the same substrate,using a photolithography method and a reactive ion etching method. Theoptical circuit of the broadband wavelength multiplexing anddemultiplexing filter 50 is a circuit shown in FIG. 11 which is designedsuch that optical signals in the wavelength bands of 1.31 μm and 1.49are output from the through port and an optical signal in the wavelengthband of 1.55 μm is output from the cross port. Then, an over clad filmmade of quartz-based glass is formed by using the flame hydrolysisdepositing method again, thereby forming the optical splitter 120.

Example 7

As an example 7 of the seventh embodiment, FIG. 22 shows the wavelengthcharacteristic of the transmittance of the splitter 120 shown in FIG.21. Further, the design values of each optical circuit are as follows.In the broadband wavelength multiplexing and demultiplexing filter 50,ΔL₀ of the second phase part is 3.16 μm, ΔL₁ of the phase part is 10.81μm, the pitch between the directional couplers is 13.6 μm, and thelength of the coupling part of the directional coupler is 0 μm. In FIG.22, ‘Th’ means the output of the through port, and ‘Cr’ means the outputof the cross port.

As can be seen from FIG. 22, in the wavelength bands of 1.31 μm (1.26 to1.36 μm) and 1.49 μm (1.48 to 1.50 μm), the insertion loss is equal toor less than 2.0 dB, and in the wavelength band of 1.55 μm (1.55 to 1.56μm), the insertion loss is equal to or less than 11.0 dB. In addition,as can be seen from FIG. 22, an optical signal in the wavelength band of1.55 μm is split into eight optical signals having the same intensity,and the split optical signals in the wavelength band of 1.55 μm and theoptical signals in the wavelength bands of 1.31 μm and 1.49 μm aremultiplexed.

According to the third to seventh embodiments, an optical waveguidecircuit is formed on a substrate to form a broadband wavelengthmultiplexing and demultiplexing filter. In this way, it is possible toeasily form a broadband wavelength multiplexing and demultiplexingfilter having a wavelength characteristic according to design by thefollowing circuit structure.

That is, in the optical waveguide circuits of the third to seventhembodiments, one or more first phase-part-intervention-typepoint-symmetrically connected optical interferometer circuits, eachformed by accurately point-symmetrically connecting two equalMach-Zehnder optical interferometer circuits in series on a substrate,are connected to each other in series. The Mach-Zehnder opticalinterferometer circuit is a phase-part-intervention-typepoint-symmetrically connected optical interferometer circuit in whichtwo equal optical couplers are accurately point-symmetrically connectedto each other in series with a phase part interposed therebetween. Sincethe optical coupler comprises the same directional coupler and thesecond phase part having a different length (phase amount) from thatbetween the optical couplers, the optical coupler has high isolationwith respect to outputs from both the through port and the cross port,and thus it is possible to form an optical waveguide without using alens system. Therefore, it is possible to reduce manufacturing costs.

Eighth Embodiment

Next, a circuit of an eighth embodiment according to the presentinvention will be described. This circuit is an optical coupler circuithaving an optical signal multiplexing and demultiplexing function,similar to the seventh embodiment. As shown in FIG. 28, the opticalsplitter having the optical signal multiplexing and demultiplexingfunction that is used in the current B-PON system is formed byconnecting a filter module 31 having a dielectric multi-layer filter,functioning as an optical signal multiplexing and demultiplexing device,to an optical splitter module 31 using a melting coupler (a meltingoptical fiber coupler). The filter module 30 and the optical splittermodule 31 are connected to each other by connecting optical fibers 33and 34 formed at light input and output units thereof using a fusingsplicing portion 32. In addition, a module having a plane lightwavecircuit (PLC) may be used as the optical splitter module.

However, in a conventional example, since the dielectric multi-layerfilter module 30 and the optical splitter module 31 are connected toeach other by fusion-splicing the terminals of the optical fibers 33 and34, the size of a module is extremely increased. In particular, in theconventional example, when an image distribution number increases, thestructure of a module is complicated, and a size, an accommodationspace, and a module cost substantially increase.

The optical splitters with the optical signal multiplexing anddemultiplexing function according to the seventh and eighth embodimentsof the present invention have a structure for solving theabove-mentioned problems. That is, the optical splitter with the opticalsignal multiplexing and demultiplexing function according to each of theseventh and eighth embodiment has a small size and a simple structurecapable of flexibly coping with the increase of the image distributionnumber, and also has a function of multiplexing and demultiplexing thewavelengths of signals transmitted and received in high-speed datacommunication and a function of equally distributing image signals.

FIG. 23 shows the optical splitter with the optical signal multiplexingand demultiplexing function according to the eighth embodiment of thepresent invention. As shown in FIG. 23, the optical splitter with theoptical signal multiplexing and demultiplexing function according to theeighth embodiment has an optical waveguide circuit 10 formed on asubstrate 15. The optical waveguide circuit 10 has an optical splitter 3for splitting an optical signal input to a light input port 1 arrangedat one end of the optical waveguide circuit 10 into a plurality ofoptical signals having the same intensity and for outputting them from aplurality of light output ports 4.

In the present embodiment, the optical splitter 3 is an eight-branchsplitter or a Y-branch splitter. An optical signal input from the lightinput port 1 is split into eight optical signals, and the split eightsignals are respectively output from the light output ports 4. Inaddition, the optical splitter 3 also has a function of multiplexingsignals input from the light output ports 4 and of outputting themultiplexed signal from the light input port 1.

Further, the optical waveguide circuit 10 has a plurality of the opticalsignal multiplexing and demultiplexing devices 5 arranged in parallel.Each optical signal multiplexing and demultiplexing device 5 comprisestwo light input ports 2 and 7 and has at least a function ofmultiplexing optical signals having different wavelengths that are inputfrom the light input ports 2 and 7.

The light input port 7 of each optical signal multiplexing anddemultiplexing device 5 is connected to the corresponding light outputport 4 of the optical splitter 3, and the other input port 2 of eachoptical waveguide multiplexing and demultiplexing device 5 is arrangedat one end of the optical waveguide circuit 10 to be disposed parallelto the light input port 1 of the light splitter 3. In addition, amultiplexed optical signal output port 8 of each optical signalmultiplexing and demultiplexing device 5 is arranged at an end portion(here, the other end) other than the end portion where the light inputports 1 and 2 are arranged in the optical waveguide circuit 10.

Further, each optical signal multiplexing and demultiplexing device 5also has a function of multiplexing a plurality of optical signalshaving different wavelengths input from the multiplexed optical signaloutput port 8 and of respectively outputting the multiplexed opticalsignals from the light input ports 2 and 7.

FIG. 2 schematically shows the structure of each optical signalmultiplexing and demultiplexing device 5 according to the presentembodiment. Since the optical signal multiplexing and demultiplexingdevice 5 is the same as the point-symmetrically connected opticalinterferometer circuit 5 constituting the light input side circuit shownin FIG. 1, a description thereof will be omitted for the simplicity ofexplanation.

In the optical signal multiplexing and demultiplexing device 5, asdescribed above, the transmittance T_(CR) of light output from the crossport is obtained by Expressions (1), (2), and (3).

The optical signals in the wavelength bands of 1.31 μm, 1.49 μm, and1.55 μm are input to the IN port. Then, among them, the optical signalsin the wavelength bands of 1.31 μm and 1.49 μm are output from thethrough port, and the optical signal in the wavelength band of 1.55 μmis output from the cross port.

In this case, when C is zero for the respective central wavelengths oftwo of the three wavelength bands, T_(CR) becomes zero for thesewavelength bands. Therefore, transmittance when optical signals in thesewavelength bands are output from the through port is 1 from therelationship 1−T_(CR)=1. Thus, two specified wavelengths are output fromthe through port.

Here, since C is zero for two wavelengths, the relationship cos²(ΔΨ/2)=0 is established in Expression (2). For example, ΔL_(C) isdetermined to satisfy ΔΨ/2=(2m+3)π at the wavelength of 1.31 μm andΔΩ/2=(2m+1) it at the wavelength of 1.49 μm.

Next, at a wavelength of 1.55 μm, T_(CR) is 1, and the cross port 19 isbroadband. Therefore, C is set to about 0.5 for the wavelength of 1.55μm. The K value of the directional coupler 6 is calculated to satisfythe above-mentioned relationship.

Finally, since the wavelength band of 1.31 μm is the widest transmissionband, it is possible to widen a wavelength band by setting the value ofK to about zero or 1 (100%) at the wavelength of 1.31 μm, which is thecentral wavelength thereof.

For example, when manufacturing a quartz-based optical waveguide circuithaving a relative index difference Δ of 0.45% and a size of 7.5×7.5 μm,the value of ΔL_(C) calculated according to the above-mentioned processis 7.71 μm, and the coupling efficiency K of the directional coupler atthe wavelength of 1.31 μm is about zero or 1. In addition, the couplingefficiency K of the directional coupler at the wavelength of 1.55 μm isabout 0.77.

Any type of directional coupler 6 will be used if satisfying theabove-mentioned conditions. However, in general, the directional couplerhaving a short coupling part in length is manufactured with a low errorrate. Therefore, it is preferable that the length of the coupling partof the directional coupler 6 shown in FIG. 3 be short.

That is, for example, the length of the coupling part of the directionalcoupler 6 satisfying the above-mentioned conditions is 924 μm or 2184μm. As shown in FIGS. 4A and 4B, even when the length of the couplingpart of the directional coupler 6 is 924 μm (FIG. 4A) or 2184 μm (FIG.4B), the above-mentioned conditions are satisfied. In this case, it ispreferable that the length of the coupling part of the directionalcoupler 6 be 924 μm since it is expected that manufacturing errorsdecrease.

Further, in the eighth embodiment, in each optical signal multiplexingand demultiplexing device 5, the directional coupler 6 is formed suchthat the length of the coupling part thereof is 924 μm. In addition, inthe wavelength characteristics shown in FIG. 4A, the coupling efficiencyK of the directional coupler 6 is set to about 1 at a wavelength of 1.31μm, and a differential coefficient dK/dλ of the coupling efficiency Kwith respect to a wavelength of 1.55 μm is set to satisfy dK/dλ<0. Theoptical signal multiplexing and demultiplexing device 5 according to theeighth embodiment is formed as described above, and thus can realizegood characteristics capable of satisfying characteristics required forthe respective wavelength bands of 1.31 μm, 1.49 μm, and 1.55 μm.

Since the broad band multiplexing and demultiplexing device (thebroadband optical signal multiplexing and demultiplexing filter) can beconstructed as described above and the optical signal multiplexing anddemultiplexing device 5 having the above-mentioned structure isconnected to the optical splitter 3 in the eighth embodiment, it ispossible to form an optical splitter having an optical signalmultiplexing and demultiplexing function for performing opticalmultiplexing and demultiplexing on a desired wavelength with a lowerloss.

For example, in the eighth embodiment, when optical signals in awavelength band of 1.55 μm are input to the light input port 1 of theoptical splitter 3, the optical signals are split by the opticalsplitter 3, and the split optical signals are input to the opticalsignal multiplexing and demultiplexing device 5 through the light inputport 7 of the optical signal multiplexing and demultiplexing device 5.Since the multiplexed optical signal output port 8 is a cross port withrespect to the light input port 7, it is possible to output opticalsignals in the wavelength band of 1.55 μm from the demultiplexed opticalsignal output port 8 with low loss by the above-mentionedcharacteristics of the optical signal multiplexing and demultiplexingdevice 5.

When optical signals in a wavelength band of 1.31 μm and a wavelengthband of 1.49μ are input from the light input port 2 of the opticalsignal multiplexing and demultiplexing device 5, the optical signals inthese wavelength bands of 1.31 μm and 1.49 μm and the optical signal inthe wavelength band of 1.55 μm are multiplexed with each other by theoptical signal multiplexing and demultiplexing device 5. Since themultiplexed optical signal output port 8 is a through port with respectto the light input port 2, it is possible to output the optical signalsin the wavelength bands of 1.31 μm and 1.49 μm from the multiplexedoptical signal output port 8 with low loss by the above-mentionedcharacteristics of the optical signal multiplexing and demultiplexingdevice 5. As a result, it is possible to output the optical signalobtained by multiplexing the optical signals in the wavelength bands of1.31 μm, 1.49 μm, and 1.55 μm from the multiplexed optical signal outputport 8 with low loss.

Moreover, contrary to the above, when the optical signals in thewavelength bands of 1.31 μm, 1.49 μm, and 1.55 μm are input from themultiplexed optical signal output port 8, through a course contrary tothe above, the optical signals in the wavelength bands 1.31 μm and 1.49μm can be output from the light input port 2 of the optical signalmultiplexing and demultiplexing device 5 formed at one end of theoptical waveguide circuit 10 with low loss, and the optical signal inthe wavelength of 1.55 μm can be output from the light input port 1 ofthe optical splitter 3 with low loss.

Further, in the eighth embodiment, as shown in FIG. 23, it is possibleto construct a compact circuit, similar to the circuit shown in FIG. 21,by arranging the optical signal multiplexing and demultiplexing device 5in an array shape and by connecting it to the optical splitter 3 on thesame substrate using the waveguide. That is, in the eighth embodiment, amodule is 8 mm wide, 60 mm long, and 70 mm high, which is about half ofthe conventional module that is 20 mm wide, 120 mm long, and 20 mm high.Therefore, it is possible to decrease the size of a module.

Furthermore, the optical waveguide circuit 10 according to the eighthembodiment has a cross waveguide therein. However, it is possible tominimize the leakage of light to other channels by properly designingthe layout of an optical circuit (for example, a crossed angle is set tobe larger than 20°) (which is similarly applied to the circuit shown inFIG. 21).

Moreover, the size reduction effect (achievement of a small size) by theseventh and eighth embodiments is more remarkably obtained when thedistribution number of signals increases. As a result, an accommodationspace is reduced, and it is possible to cope with an increase in thenumber of channels without a sharp increase in cost.

Example 8

As an example 8 of the eighth embodiment, an optical splitter with afunction of multiplexing and demultiplexing an optical signal ismanufactured as follows. First, an under clad film made of quartz-basedglass and a TiO₂-doped core film are formed on a silicon substrate usinga flame hydrolysis depositing method. At that time, the relative indexdifference Δ of the core to the clad is set to 0.45%, and the thicknessof each film is set to 7.5 μm.

Subsequently, an optical circuit pattern is transferred to the corethrough a photo mask on which a circuit of the optical splitter with anoptical signal multiplexing and demultiplexing function shown in FIG. 23is drawn, by a photolithography method and a reactive ion etchingmethod. Then, an over clad film made of quartz-based glass is formedusing the flame hydrolysis depositing method again. In this way, theoptical splitter with an optical signal multiplexing and demultiplexingfunction is manufactured by forming the circuit structure shown in FIG.23 with the optical waveguide of the core.

For example, the width of the core constituting the optical waveguidecircuit is 7.5 μm, and the thickness of the core is also 7.5 μm. ΔL_(C)of the directional coupler is set to 7.71 μm, the pitch between thedirectional couplers 6 (as shown in FIG. 3, the distance between thecenter of the first optical waveguide 11 and the center of the secondoptical waveguide 12) is set to 11.1 μm, and the length of the couplingpart of the directional coupler is set to 924 μm.

Then, the optical splitter with an optical signal multiplexing anddemultiplexing function is cut into a predetermined size (for example, awidth of 4.7 mm, a length of 47 mm, and a height of 1 mm) by a dicingmachine. Subsequently, as shown in FIG. 24, optical fiber arrays 20 and21 are connected to both ends of the optical waveguide circuit, and theyare integrated into a package, thereby forming an optical module. Themodule has a size of, for example, a width of 8 mm, a length of 60 mm,and a height of 7 mm. Further, the structure of the optical waveguidecircuit 10 is not shown in FIG. 24. In FIG. 24, reference numeral ‘26’indicates an upper plate, and reference numerals ‘22’, ‘23’, ‘24 ’, and‘25’ indicate connection end surfaces. In addition, the connection endsurfaces 22 and 23 opposite to each other are connected, and theconnection end surfaces 24 and 25 opposite to each other are connected.

The characteristics of the optical splitter with an optical signalmultiplexing and demultiplexing function according to the example 8 isas follows. That is, for the respective optical signals havingwavelengths of 1.31 μm and 1.49 μm that are input from the light inputport 2, the insertion loss is about 0.7 dB, and a PDL (polarizationdependence loss) is about 0.2 dB. The insertion loss of the opticalsignal having a wavelength of 1.55 μm input from the light input port 1is about 10.7 dB, and the PDL thereof is about 0.2 dB. In addition, thereflection attenuation amount of optical signals input from therespective light input ports 1 and 2 is smaller than 40 dB. Further, thetransmission loss characteristic (wavelength characteristic) of theoptical signal input from the light input port 2 is represented by acharacteristic line a in FIG. 25.

As can be seen from these results, the example 8 has characteristicscorresponding to design values, and thus the validity of design isestablished.

Furthermore, the broadband wavelength multiplexing and demultiplexingfilter of the present invention is not limited to the respectiveembodiments, and various modifications and changes can be made. Forexample, in the examples 1 and 2, the relative index difference Δ of thecore to the clad is set to 0.45% or 0.3%. However, the relative indexdifference Δ may be other values. For example, the value may be 0.80%.

Moreover, the length of the coupler part of the directional coupler 6 ofthe Mach-Zehnder optical interferometer circuit 13 and the pitch betweenthe directional couplers are not limited to specific values, but may beproperly set to correspond to the required wavelength characteristic.

Further, in the first and second embodiments, the light input sidecircuit 1 and the first light output side circuit have thepoint-symmetrically connected optical interferometer circuits 5 and 7,respectively. However, at least one of the light input side circuit 1and the first light output side circuit 2 may be formed by connectingtwo or more point-symmetrically connected interferometer circuits inseries.

Furthermore, the second light output side circuit 3 has the two-stageMach-Zehnder optical interferometer circuits 13 c and 13 d in the firstembodiment, and has the one-stage point-symmetrically connectedinterferometer circuit 8 and the Mach-Zehnder optical interferometercircuit 13 d connected to the next stage thereof in the secondembodiment. However, the number of stages of the Mach-Zehnder opticalinterferometer circuit 13 and the number of stages of thepoint-symmetrically connected optical interferometer circuit 8constituting the second light output side circuit 3 are not particularlylimited thereto, but may be properly set.

Moreover, in the respective embodiments, the broadband wavelengthmultiplexing and demultiplexing filter comprises one light inputterminal 17 and two light output terminals. However, a broadbandwavelength multiplexing and demultiplexing filter having an alternativestructure of the present invention can be formed by arranging aplurality of broadband wavelength multiplexing and demultiplexing filtercircuits each having one light input terminal 17 and two light outputterminals in an array shape. The broadband wavelength multiplexing anddemultiplexing filter having the alternative structure has a pluralityof light input terminals 17 and light output terminals corresponding tothe number of the light input terminals 17.

According to this structure, it is possible to realize a broadbandwavelength multiplexing and demultiplexing filter capable of reliablymultiplexing and demultiplexing optical signals having a larger numberof wavelengths with high isolation, by arranging a plurality ofbroadband wavelength multiplexing and demultiplexing filters capable ofexhibiting the above-mentioned excellent effects.

Further, as described above, optical signals having a plurality ofwavelength input from one light input terminal 17 are demultiplexed, andthe demultiplexed optical signals are output from the through port andthe cross port. To the contrary, optical signals having differentwavelengths may be respectively input from the through port 28 and crossport at the light output side, and output from the light input terminal17 through a course opposite to the above.

Furthermore, in the eighth embodiment, the relative index difference Δof the core to the clad is set to 0.45%. However, the relative indexdifference Δ may be other values. For example, the value may be 0.80% or0.30%.

Moreover, in the seventh and eighth embodiments, the branch number ofeach of the optical splitters 122 and 3 is eight (an eight-branchsplitter). However, the branch number of each of the optical splitters122 and 3 is not limited thereto, and may be properly set. For example,the optical waveguide circuit 10 can be formed of a four-branchsplitter, a sixteen-branch splitter, or a thirty-two-branch splitter.

Further, in the above-mentioned embodiments, the length of the couplingpart of the directional coupler 6 of the optical signal multiplexing anddemultiplexing device 5 is set to 924 μm. However, under the wavelengthmultiplexing and demultiplexing conditions of the above-mentionedembodiments, the length of the coupling part may be, for example, 2184μm or other values, and may be properly set corresponding to thewavelength multiplexing and demultiplexing conditions (for example,multiplexing and demultiplexing wavelengths) required for the opticalsplitter with an optical signal multiplexing and demultiplexingfunction.

Furthermore, in the above-mentioned embodiments, the multiplexed opticalsignal output port 8 of the optical signal multiplexing anddemultiplexing device 5 is provided at the other end side of the opticalwaveguide circuit 10. However, the multiplexed optical signal outputport 8 may be formed at a different end from an area where the lightinput ports 1 and 2 are arranged.

Moreover, in the above-mentioned embodiments, the optical signalmultiplexing and demultiplexing device 5 is formed by connecting twoMach-Zehnder optical interferometer circuits 13 in series. However, theoptical signal multiplexing and demultiplexing device 5 may be formed ofone Mach-Zehnder optical interferometer circuit 13. Therefore, when twoMach-Zehnder optical interferometer circuits 13 having the connectionstructure as in the above-mentioned embodiments are provided to form theoptical signal multiplexing and demultiplexing device 5, it is possibleto widen the pass band width of a plurality of wavelengths multiplexedand demultiplexed by the optical signal multiplexing and demultiplexingdevice 5, which is more preferable.

INDUSTRIAL APPLICABILITY

As described above, a broadband wavelength multiplexing anddemultiplexing filter and an optical splitter with an optical signalmultiplexing and demultiplexing function according to the presentinvention are used for various purposes of multiplexing anddemultiplexing (distributing) optical signals having a plurality ofwavelengths in an optical communication field. In particular, thesedevices are suitably applied to a process of multiplexing anddemultiplexing optical signals in a B-PON system.

1. An optical splitter with an optical signal multiplexing anddemultiplexing function comprising: an optical waveguide circuit formedon a substrate, wherein the optical waveguide circuit comprises: anoptical splitter for splitting an optical signal input from a lightinput port provided at one end of the optical waveguide circuit into aplurality of optical signals having the same intensity and foroutputting them from a plurality of light output ports; and a pluralityof optical signal multiplexing and demultiplexing devices arranged inparallel to each other, each being provided with two light input portsand having a function of multiplexing optical signals having differentwavelengths input from the light input ports, wherein one input port ofeach of the optical signal multiplexing and demultiplexing devices isconnected to the corresponding light output port of the opticalsplitter, the other light input port of each of the optical signalmultiplexing and demultiplexing devices is provided at one end side ofthe optical waveguide circuit to be parallel to the light input port ofthe optical splitter, a multiplexed optical signal output port of eachof the optical signal multiplexing and demultiplexing devices isprovided at an end portion side other than a region where the lightinput port of the optical waveguide circuit is provided, and filtercircuits having a two-stage structure are connected in series to theoptical multiplexing and demultiplexing devices.
 2. The optical splitterwith an optical signal multiplexing and demultiplexing functionaccording to claim 1, further comprising: a first optical waveguide anda second optical waveguide provided in parallel to the first opticalwaveguide with a gap therebetween, wherein two Mach-Zehnder opticalinterferometer circuits, each having directional couplers formed byarranging the first and the second optical waveguides closely to eachother with a gap in a lengthwise direction of the optical waveguidestherebetween, are connected in series to each other to form an opticalsignal multiplexing and demultiplexing device, an arrangement pitchbetween the directional couplers in one Mach-Zehnder opticalinterferometer circuit is equal to that in the other Mach-Zehnderoptical interferometer circuit, a length of a phase part of the firstoptical waveguide is larger than that of the second optical waveguide bya set length, in the one Mach-Zehnder optical interferometer circuit,and a length of a phase part of the second optical waveguide is largerthan that of the first optical waveguide by the set length, in the otherMach-Zehnder optical interferometer circuit.